Fracture sealing and fluid overpressures in limestones of the Jabal Akhdar dome, Oman mountains


Christoph Hilgers, Geologie-Endogene Dynamik, RWTH Aachen, Lochnerstr. 4–20, D-52056 Aachen, Germany.
Email: Tel: + 49  241  809 5723. Fax: + 49  241  809 2358.


Fractures are important conduits for fluid flow in the Earth's crust. To better understand the spatial and temporal relations among fracturing, fracture sealing, and fluid flow, we have studied fractures, faults, and veins in a large dome (Jabal Akhdar) in the Oman mountains. Our work combines the results of meso- and microstructural analyses and stable isotope analyses. Seven generations of fractures and veins have been identified in the carbonate-dominated dome. The earliest generations of veins developed during extension and subsidence of the Mesozoic basin. These veins formed in the inclined segments of bedding-parallel stylolites and in extensional fractures that are perpendicular to bedding (#1 and #2, respectively). These extension-related veins are truncated by bedding-parallel veins (#4) that formed during top-to-north bedding-parallel shear of both the northern and southern limbs of the dome. These veins are consistent with a change in stress regime and may be related to an earlier generation of strongly deformed pinch-and-swell veins (#3) that are exposed locally on the southern limb of the dome. Normal faults contain a set of en-echelon tension gashes (#5) and veins emplaced in dilational jogs along the fault planes (#6). In the northern part of the dome, veins (#7) associated with thrusts post-date the normal faults. Samples of veins and their host rocks were analyzed to provide information on fluid-rock interaction in the dome and the scale(s) of fluid movement. Oxygen isotope values range from +16.2 to +29.3‰; carbon isotope values range from 0 to +3.6‰. The results of the structural and isotopic analyses are consistent with the early veins (#2–#5) having precipitated from overpressured fluid in a isotopically rock-buffered system. During normal faulting (#5 and #6), a more open system allowed external fluid to infiltrate the dome at drained conditions and precipitate the youngest sets of veins (#6 and #7).